122 lines
5.7 KiB
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122 lines
5.7 KiB
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<HTML>
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<HEAD>
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<TITLE>A Simple Extractor and Inserter for the Example</TITLE>
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<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
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<A HREF="32-2.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="32-4.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
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<H2>32.3 A Simple Extractor and Inserter for the Example</H2>
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<P>Following the read and write conventions of iostreams, we would insert and extract the date object like this:</P>
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<UL><PRE>
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date birthday(2,6,1952);
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std::cout << birthday << '\n';
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</PRE></UL>
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<P>or </P>
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<UL><PRE>
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date aDate;
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std::cout << '\n' << "Please, enter a date (day month year)"
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<< '\n';
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std::cin >> aDate;
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std::cout << aDate << '\n';
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</PRE></UL>
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<A NAME="idx781"><!></A>
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<P>For the next step, we would implement shift operators as inserters and extractors for <SAMP>date</SAMP> objects. Here is an extractor for class <SAMP>date</SAMP>:</P>
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<UL><PRE>
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template<class charT, class Traits>
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std::basic_istream<charT, Traits>& //1
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operator>> (std::basic_istream<charT,Traits>& is, //2
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date& dat) //3
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{
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is >> dat.tm_date.tm_mday; //4
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is >> dat.tm_date.tm_mon;
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is >> dat.tm_date.tm_year;
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return is; //5
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}
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>The returned value for extractors (and inserters) is a reference to the stream, so that several extractions can be done in one expression.
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<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>The first parameter usually is the stream from which the data shall be extracted.
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<TR VALIGN="top"><TD><SAMP>//3</SAMP></TD><TD>The second parameter is a reference, or alternatively a pointer, to an object of the user-defined type.
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<TR VALIGN="top"><TD><SAMP>//4</SAMP></TD><TD>In order to allow access to private data of the date class, the extractor must be declared as a friend function in class <SAMP>date</SAMP>.
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<TR VALIGN="top"><TD><SAMP>//5</SAMP></TD><TD>The return value is the stream from which the data was extracted.
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</TABLE>
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<A NAME="idx782"><!></A>
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<P>As the extractor accesses private data members of class date, it must be declared as a friend function to class date. The same holds for the inserter. Here's a more complete version of class <SAMP>date</SAMP>:</P>
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<UL><PRE>
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class date {
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public:
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date(int d, int m, int y);
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date(tm t);
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date();
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// more constructors and useful member functions
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private:
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std::tm tm_date;
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template<class charT, Traits>
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friend std::basic_istream<charT, Traits>
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&operator >>(std::basic_istream<charT, Traits >& is, date& dat);
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template<class charT, Traits>
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friend std::basic_ostream<charT, Traits>
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&operator <<(std::basic_ostream<charT, Traits >& os,
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const date& dat);
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};
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</PRE></UL>
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<A NAME="idx783"><!></A>
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<P>The inserter can be built analogously, as shown below. The only difference is that you would hand over a <I>constant</I> reference to a date object, because the inserter is not supposed to modify the object it prints.</P>
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<UL><PRE>
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template<class charT, class Traits>
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std::basic_ostream<charT, Traits>&
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operator << (std::basic_ostream<charT, Traits >& os,
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const date& dat)
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{
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os << dat.tm_date.tm_mon << '-';
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os << dat.tm_date.tm_mday << '-';
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os << dat.tm_date.tm_year ;
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return os;
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}
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</PRE></UL>
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<BR>
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